Resolve extra_unused_lifetimes clippy lint

error: this lifetime isn't used in the impl
       --> serde/src/de/value.rs:607:11
        |
    607 | impl<'de, 'a, E> de::EnumAccess<'de> for StringDeserializer<E>
        |           ^^
        |
        = note: `-D clippy::extra-unused-lifetimes` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes

serde/src/de/value.rs

@@ -604,7 +604,7 @@ where
 }
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-impl<'de, 'a, E> de::EnumAccess<'de> for StringDeserializer<E>
+impl<'de, E> de::EnumAccess<'de> for StringDeserializer<E>
 where
     E: de::Error,
 {